The most common separator materials are nonwoven fabrics of polyamide and/or polyolefin fibers due to the flexibility and electrolyte resistance of such fibers. Such separator materials serve to prevent contact between the positive and negative electrodes and, thus, tend to prevent an undesired flow of current in electrochemical cells having a liquid alkaline electrolyte, however, without substantially impeding the passage of ions through the liquid electrolyte.
Aqueous alkaline electrolyte is used, for example, in the electrochemical cells of nickel-cadmium-cells or nickel-hydrogen and nickel-metal hydride batteries. In this case, both the positive and the negative electrodes consist of thin rollable strips of the active electrode materials. Between the two strips, the separator material consisting of nonwoven fabric of the type mentioned above is inserted and rolled up together with the electrodes. After the rolling-up step, this four-layer structure consists of a positive and a negative electrode and two layers of separator material. The coil thus produced is inserted into a cup-shaped housing of nickel-plated steel sheet, which is then filled with highly concentrated aqueous potassium hydroxide solution. The electrolyte must be absorbed quickly under these circumstances by the nonwoven separator material and the electrolyte must be deposited in the pores of the nonwoven separator material.
Unfortunately, polyamide or polyolefin-fiber separator materials are by their very nature hydrophobic and, therefore, by themselves cannot satisfy the important requirement of rapid and good wettability to the extent, as mentioned above, required by the electrolyte. Poor wettability leads to disturbances in the manufacture of the cells, since the electrolyte added cannot be absorbed sufficiently rapidly and, thus, is not sufficiently distributed within the cell. The result is that upon subsequent addition of electrolyte the electrolyte will overflow and soil the production equipment. Also, upon subsequent operation of the battery, gas bubbles will accumulate on and in a separator material having poor wettability, with the gas bubbles thoroughly impeding the passage of ions and, thus, perceptibly reducing the performance of the battery and even blocking it. In the case of gas-tight electrochemical cells, under these circumstances there is also produced an excess pressure that can lead to a bursting of the cell housing. Several tests were carried out to make separator materials of basically hydrophobic nonwoven materials sufficiently hydrophilic for use in an electrochemical cell.
Thus, for example, U.S. Pat. No. 3,947,537 describes a treatment of the fiber surface with a surface-active agent, a surfactant.
DE-A 25 43 149 mentions the addition of hydrophilic substances at the polymer melt stage, that is, at the point of spinning the filaments which are to be used as separator materials.
Neither method has proven useful since any introduction of additional chemicals into the sensitive chemical system of an electrochemical cell can lead to disturbances, the causes of which can be reduced only with difficulty and with the use of numerous different chemical substances. It is therefore preferable to construct separators only from precisely defined fiber materials and to use only those hydrophilizing additives which do not cause any disturbances in the operation of the electrochemical cell.